In the automatic testing of printed circuit boards, conventional spring-loaded test probes are typically mounted on a test fixture, and the circuit board under test is held in pressure contact with the spring probes to provide electrical contact between diagnostic or testing equipment and circuits on the board under test. Each test fixture typically contains a base or probe plate having an array of spring probes dedicated to the board under test. Identical circuit boards are then aligned with the fixture and tested, one at a time, to determine whether there are any defects in the circuits printed on each board.
During testing, the spring probes must be correctly aligned with test points in the circuits printed on the board under test. This requires correct alignment of the board on the fixture so that the spring probes are automatically registered with corresponding test points in the circuit array printed on each board. In order to align the board with the probes, the customary practice has been to first drill tooling pin holes in the circuit board. Drilling is done by the board manufacturer, and usually a pair of tooling pin holes are drilled in diagonally opposite corners of the board. During testing, tooling pins in opposite corners of the fixture are positioned in corresponding tooling pin holes to automatically align the board with the array of test probes on the fixture. This procedure requires that the tooling pin holes be drilled accurately in the board by the board manufacturer, so that when each board is later mounted on the tooling pins, the test probes will automatically register with the circuits printed on the board when the test probes are brought into pressure contact with the board during testing.
Printed circuit boards made by a given manufacturer are usually produced in lots, and boards within a given lot will tend to produce a certain misalignment, from tolerance error, when boards within that lot are mounted on a fixture. Alignment tolerances will tend to vary from lot to lot with a given manufacturer. The same board made by a different manufacturer also tends to have different tolerance variations and misalignment errors from lot to lot.
Printed circuit boards typically have a pair of "fiducial marks" printed in the margin of the board at known reference positions relative to the circuit array or "artwork" printed on the board. The fiducial marks are printed on the board by the board manufacturer, and the fiducial marks are used as a guide for the manufacturer's drilling of the tooling pin holes at locations on the board that will automatically align (hopefully) the board to the test probes when the board is mounted on the tooling pins.
There are drawbacks to using these procedures for aligning a printed circuit board with a test fixture. Mechanical tolerances are involved in (1) positioning of the drilling tool in the drilling machine, (2) positioning of the tooling pin holes drilled in the printed circuit board with respect to the artwork on the board, and (3) positioning of the tooling pins on the fixture with respect to the tooling pin holes in the board. Because of these tolerance accumulations, the test probes often do not precisely register with the test points in the circuit array printed on the board. This problem is magnified by tolerance variations in board lots from different manufacturers. The problem also is compounded by advances in the art of printed circuit board technology, where printing of circuits with finer lines and narrower circuit spacings requires more accurate registration of the test probes with the circuits under test. At the present time, for example, it is common for test probes to be misaligned within a range of 0.005 to 0.008 inch in a typical test fixture; more accurate alignment is desirable, and the present invention provides techniques for achieving greater alignment accuracy.